Keywords to describe people counting from a camera?
I usually start with some papers and look at the references:


Counting people using video cameras
Sheng-Fuu Lin, Jaw-Yeh Chen, Hung-Xin Chao, Estimation of Number of People in Crowded Scenes Using Perspective Transformation,  IEEE Transactions on Systems, Man and Cybernetics, November 2001, Part A, Vol. 31, Issue 6, pp. 645-654. 
A.  C.  Davies,  J.  H.  Yin,  and S.  A.  Velastin,  Crowd monitoring using image processing,  Electronics  &amp; Communication Engineering Journal, February  1995, Vol. 7, Issue 1, pp. 37-47.  


By this technique you only find older work.

To find more recent articles, use Google scholar and find who cites the articles you investigated.

Always write them down (e.g. in your bibtex file) to keep track of what you looked at already.

Good luck!